300Bs - does leaving them on shorten their life?

No it's not a good idea. You need to be there or at least in listening range.. It's just the nature of the beast. Those are exposed bulbs, Valves, or tubes.. Be wise.

They don't always fail and keep it in the glass, get my meaning..

I've seen an valve fire or two. LOL TRUST ME the first one is an eye opener.. I was 15.. I learned QUICK.. Almost burned down my mothers house. I had to pay for the carpet I destroyed.. That wasn't funny. Mac MC20s and a MAC 30 Kit.. BLEW a 6L6.. caught the carpet on fire..

I was trying to power a Jensen Imperial Sub.. It worked great.. UNTIL...
My mother was a saint...

regards

I have NEVER heard a manufacture tell anyone to leave the "Power Valves" on.. EVER.. Then again, what do I know..

The little signal valves, I've seen stay on for months at a time..  Some designs are VERY low voltage and the valves last 15-20,000 hours..  I've not seen power valves do that.. 5,000 hours is really a push. 2-3000 is more like it..

I've never seen a signal valve erupt, and breach the glass, rectifier and power valves, I have...

What about some of the hand made valves.. 4K+ for a pair. Rebuildable.
Maybe I should start making them out of old pint and quart canning jars..

Gears are grinding...

Regards